Method and apparatus for indicating radioactivity percentage ratios



May 31, 1955 w. c. DAvlDoN METHOD AND APPARATUS Foa INDTCATING RAnIoAcTTvITT PERCENTAGE RATIos 2 Sheets-Sheec l Filed April 14, 1954 56a Ier' FlELE Sca Zelf' May 31, 1955 w. c. DAvlDoN 2,709,754

METHOD AND APPARATUS FOR INDICATING RADIOACTIVITY PERCENTAGE RATIOS Filed April 14, 1954 2 Sheets-Sheet 2 ME'IHGD AND APPARATUS FR WDCATNG RADIACTWTY PERCENTAGE RATIS William tC. Bavidon, Chicago, lli., assigner to Nuclear Instrument and Chemical Corporation, Chicago, lll., a corporation oi illinois Application April lil, i954., Serial No. 423,133

9 Claims. (till. 25d-ud@ This invention relates to a device for the measurement of radioactivity, and more specically to a method and apparatus for indicating the percentage ratio oi' the radioactivity of one radioactive specimen to that of a standard specimen.

in numerous applications where radioactive materials are employed, it is desirable to produce an indication of the percentage ratio of the activities of two radioactive specimens, one of which is free tently employed as a standard. ln many such applications, the absolute values of the radioactivities of the arious specimens are essentially of no interest, the sole desir d information lying in the ratios of activities. Nevertheless, it has heretofore been the general custom to measure the respective radioactivities of the specimens involved in a manner capable or" indicating the absolute activities, and then calculating the relative percentage, either hy the use oi conventional arithmetical computations or by the use or tables, nomographs, or similar calculation devices.

The relative difficulty and complexity of prior art iethods and apparatus for determining the percentage ratios of radioactivities have constituted a serious impediment to the full utilization or" radioactive isotopes in many iields, particularly in iields in which computations such as the calculation of percentages are not everyday operations which are conventionally performed by workers in those elds. As a pertinent example, it is well known that radioactive iodine is a highly ei'iective tool in the diagnosis and therapy of thyroid disorder T he use of this technique has become widespread in hospitals and medical centers in which research and development in advanced methods of diagnosis and treatment constitute a substantial activity. The highly trained personnel of these institutions perform the various measurements and calculations require-fl with conventional radioactivity measuring equipment on a routine basis. But the extension of the use of radioactive iodine (and, of course, other isotopes) to everyday diagnosis and therapy by medical personnel to whom the making of such measurements and calculations constitutes a completely unfamiliar endeavor has been relatively slow, so that the impact of the new techniques of isotope diagnosis and therapy has not been as great or as rapid as had been expected.

The fact that the use of radioisotope techniques in medicine, particularly in the use of radioactive iodine, has been seriously impeded by the relative complexity of the instrumentation and ot" the interpretation of the data obtained has been recognized both lby authorities in the eld of medicine and by others interested in the promotion of public health and medical science. Accordingly, a large amount of endeavor is continually directed to the simpliication of the instrumentation required for medical use of radioactive isotopes, and of the manner of interpreting the data obtained. lt has recently been suggested that the measurements required for thyroid diagnosis and therapy employing radioactive iodine may be made completely relative in character, thus eliminating any necessity of absolute measurements. The measurefice ment most commonly involved is the measurement of absorption of iodine in the thyroid of the person under diagnosis or treatment. This measurement is made by suitably introducing into the body of the patient, orally or otherwise, radioactive iodine, usually in the form of orally administered sodium iodide ltagged with i131. After a suitable interval has elapsed, a suitable detector, such as a Geiger tube or scintillation counter, is placed in a standard position with respect to the neck of the patient, and the resultant indication is employed to determine the iodine absorbed by the patient. Because of the 8-day half-life of the iodine isotope commonly employed in this measurement, it was formerly necessary to carefully record not only the concentration of the radioactive isotope, but also the exact amount administered and the exact time of the ensuing radioactive decay. Thereupon, a relatively complex computation would be made involving all of these factors, und a conclusion reached as regards the percentage of the iodine absorbed in the thyroid of the person. the new technique recently introduced, practically all ot these perations and records are eliminated. This technique lies in the preparation of duplicate specimens at the time of administration of the iodine, one being administered to the patient and the other being merely set aside. /t any time that it is desired to make a measurement of the absorption of the iodine by the thyroid, there are made measurements of the radioactivity obtained from the thyroid, as transmitted through the tissue of the neck, and also or" the radioactivity of the duplicate specimen previously se aside, as transmitted through a suitable thickness of a plastic or other material sir ating h nan tissue. The ratio of these two activities thus consti "es a measurement or" the percentage absorption of the iodine in the thyroid, irrespective or the absolute value of the original activity or the absolute value of the size of the duplicate specimens, or of the amount of radioactive decay which has occurred prior to the measurement.

The improvement described above in 'the use oiradioactivity in thyroid diagnosis and treatment, although constituting a great forward Step, has still not stimulated the spreading of the use of radioactive iodine to t e full extent of its potential beuelit to the medical profes-sion and to the public. The simplest instrument n heretodevised for this purpose has employed as toe indicator therefor an integrating counting-rate meter tnploying a needle indicator. As is well known, a counting-rate meter is subject to the diliiculty that its ref ig constantly varies over a range due to statistical variations in the occurrence of the radioactive emissions under measurement. The wavering ci the needle of a counting-rate meter is a familiar phenomenon to persons accustomed to using such instruments, who have learned by experience to average the reading of the wavering needle by eye, and thus to produce a satisfactory measurement. As is well known, if the integrating time constant of the meter is made suicientiy long, the statistical iiuctuation of the meter needle may be minimized, 1`ut only at the expense of requiring an inordinate period for the needle to reach its equilibrium position. Thus the countingrate meters which have heretofore been employed for this purpose still do not meet the needs of ordinary medical personnel who do not use uch equipment on a routine basis, and find it a disturbing task to assign a fue l reading to a meter whose. needle is wave o er a substantial range in accordance with statisti iiuctuations of the quantity being' measured.

Sealers or other registers have a distinct advantage over countering-rate meters in producing an an :ation a ilat numerical vaiue which does not rely on subjective interpretation. Prior to the present invention, there ha been devised a satisfactory method and apparatus employing scalers or other register devices giving a xed digital reading in the indication of radioactivity percentage ratios, and accordingly there had not been devised a suitable method and apparatus for the making of such measurements by persons not having a high degree of skill and experience in making measurements of this general character.

The present invention provides a method and apparatus for indicating the percentage ratio of the radioactivity of a radioactive specimen to that of a standard sepecimen which are extremely simple to perform and to operate, and the employment of which makes possible the use of radioactivity techniques, particularly the use of radioactive iodine in thyroid diagnosis and therapy, by medical personnel having little training or experience in the operation or reading of instruments for the measurement and detection of radioactivity. The method and apparatus of the invention will of course find many routine applications in other fields of endeavor, both scientific and industrial, where the potentialities of radioactive isotopes have have not yet been fully realized because of the dilculties encountered by persons not trained or experienced with relatively complex radioactivity measuring instruments. Understanding of the present invention, and of the essential features of the method and apparatus which constitute the invention, will best be obtained from the description of simple embodiments thereof which are illustrated in the drawing and described below in accordance with the patent laws.

in the drawing:

Figure 1 is a block diagram of one form of equipment with which the broader aspects of the method of the invention may be practiced;

Figure 2 is a block diagram of a system embodying the method and apparatus of the invention;

Figure 3 is a block diagram of another form of the apparatus of the invention; and

Figure 4 is a partially schematic block diagram of a particular form of the apparatus of Figure 3.

Referring rst to Figure l, it will be seen that the apparatus therein illustrated, which is suitable for the practice ot' the method of the invention in its broad aspects, is of a type which is conventional and well known. A detector Till, a Geiger tube or scintillation counter, is coupled to a sealer l2 of any desired scaling factor. The Scaler l2 is coupled to a register i4 which registers the number of pulses received from the Scaler i2. lt will of course be understood that as used herein the sealer l2 includes a suitable input amplifier and pulse-height dscriminator. The register 14 may be, for example, an electro-mechanical register, or itself may be a sealer. Whatever its construction, the register i4 must, in practicing the method of the invention, be capable of recording and indicating a number of pulses from the sealer l2 up to and including 100. The register i4 is preferably of the repetitive cycle type which recommences at zero each time its capacity is reached; as will be shown below, the employment of a register of this type having a full capacity of lil, where n is an integer greater than unity, eliminates a resetting operation in practicing the method of the invention. Thus the register le is of a type normally used in systems such as shown in Figure l, an electro-mechanical register or electronic sealer having a capacity of l0() or 1GO() counts. The scaling factor of the sealer i2 is preferably nondecimal (normally binary), since the expense and complication of decimal sealer may be avoided without introducing any complication or inaccuracy in the method. Thus the sealer l2 and register i4 may be a decimal or non-decimal sealer followed by an electr -mechanical register, or cascaded decimal Sealers, either of which combinations are well known; but the most desirable combination is a non-decimal scaler followed by a decimal sealer.

The conventional equipment shown in Figure l is employed in the method of the invention in the following fashion:

The detector lll is first exposed to the sample which constitutes the standard specimen (such as the duplicate specimen which has previously been set aside, as described above in connection with the thyroid measurements). The sealer il?. is operated until the register 14 indicates the reception of l0@ (or 1090 if desired) output pulses from the sealer l2 and the equipment is then inactivated, the time for the reaching of this count being recorded. Thereupon, the Scaler 12 and the register 14 are reset to zero if required. It will be noted that such resetting of the sealer l2 is not required if the equipment is inactivated promptly at the time set out above, since the output pulse from the sealer l2 is normally transmitted at the end of each cycle of the sealer. Liliewise, if the register lll is of the type which recycles upon reaching its capacity, and if that capacity is equal to the predetermined count, no resetting of the register 14 is required. The equipment is then reactivated while the detector tl is exposed to the sample under measurement during the same time interval as that previously recorded for the standard specimen. At this point the equipment is again inactivated. The number now indicated by the register le constitutes a direct indication (a decimal point being inserted if the number registered during the interval ixed by the standard specimen was 1G00 or a higher integral power of l0) of the percentage ratio of the radioactivity of the unknown specimen to that of the standard specimen. ln the case of the thyroid measurement discussed above, the indication of the register i3 is a direct reading of the percentage of the iodine absorbed in the thyroid.

.lt will be seen that the essence of the method as described above hes in the steps of generating pulses proportional in rate of occurrence to the radioactivity of the standard until the number thereof is equal to a multiple of lOG, generating pulses identically proportional in rate of occurrence to the radioactivity of the unknown specirnen, and registering the quotient of the number of the latter pulses occurring during the time required for the occurrence of the aforesaid number of the former pulses, divided by the multiplier of the abovementioned multiple, so that the quotient is equal to the percentage ratio of the activity of t. e unknown specimen to that of the standard. lt will also be seen that an error is introduced by disregarding the remainder which appears on the Scaler l2 at the termination of the entire operation. This error is, however, less than 1%, and is found to be of substantially no consequence in measurements made for diagnosis and therapy, as opposed to research. 0f course, if extreme accuracy is desired, the number of counts recorded on the register i4 in the rst operation may be made 1000 (or a further integral power of l0) instead of 10i), and in this event the error introduced by disregarding the remainder when the second quotient is taken is limited to 'l/i) (or less) of 1%.

ln Figure 2 is shown a novel type of apparatus for indicating this percentage ratio of the radioactivity of one radioactive specimen to that of another. ln the apparatus of this figure, a detector lll and a sealer l2 are provided in the manner previously described. The output of the sealer 12 is here coupled to a switch le which operates in response to reception of lill) pulses from the sealer 12. A second detector ida is coupled to a second scaler 12a, these elements being identical with the detector it) and sealer i2, respectively. The output of the Scaler iZa is coupled to a register ida which is the saine as the register lll of Figure l. The switch lo is responsive to the reception of l0() pulses from the sealer l2 to stop the operation of the sealer Ilia and the register la upon reception of l0@ pulses from the sealer i2.

In operating the device of Figure 2, the detector lll is exposed to the standard radioactive specimen, and the arca "fsa detector is simultaneously exposed to the sample which is under measurement, both channels being activated simultaneously. Upon reception of 1GO pulses from the sealer l2 by the switch le, the register 14a is stopped and the reading obtained therefrom is a direct reading of the percentage ratio desired. it will be understood that the switch in may be of a number of conventional types, such as a commercially available electro-mechanical indicating register having incorporated therein a switching mechanism operative upon the reception of i0() (or i000 or a further integral power of l() if extreme accuracy is desired) pulses, or a stepping switcl'i, or a predetermined count electronic sealer of any commercially available type.

The apparatus shown in Figure 2 may be subject to the objection that it is relatively expensive due to the necessity of duplicate detectors and Sealers, and also that it may be diiiicult to operate due to the necessity of adjustment of the sealers and the pulse-height selectors which are provided therein so that the sensitivities of 'the two channels are identical. in Figure 3 is shown an improved apparatus for carr ing out the method generally described above. Here the detector l() is coupled to the sealer i2 which is in turn coupled to a register-switch loa responsive to i0@ (or a further integral power of i0) pulses. T he register-switch loa is generally identical with the switch described above in connection with Figure 2, but is required to have indicating means registering the number of pulses received, a condition which is of course met by an electro-mechanical counter switch or predetermined count Scaler of the types commercially available. There are additionally provided a timer 'l and a preset time switch Eil, the timer l being coupled, as indicated by the dotted line 2,2, to the preset time switch to pr t the timing interval of the present time switch Ztl to correspond to the elapsed time during which the timer i is run. The preset che switch may, if desired, be or the commercially available type in which the timing interval is set by rotation of a suitable dial. The timer l may, in this case, be a simple motor, preferably synchronous, the coupling i: dicated by the dotted line being a mechanical coupling to the presetting mechanism of the switch 2G', so that the switch 2@ is automatically set to operate at the expiration of a time equal to the time during which the timer 18 is operated. As indicated at 2d, a switch, which may be manually operated, selectively provides power from a source of time power 26 either to the timer 18 or to the preset time switch 2d. As indicated by the dotted line 23, the timer power at is cut off or stopped when the registerswitch son receives the lGOth pulse from the sealer l2. As indicated by the dotted couplings Sli and 32, the sealer the register switch les, and the timer power source are all inactivated upon operation of the switch Ztl the time which has been preset thereon.

rlhe manner in which the apparatus shown in Figure 3 is adapted to perform the method of the invention will readily be seen. The detector itl, which produces electrical pulses proportional in rate to radioactivity, is eX- posed to the standard specimen with the switch 24 in the position to feed power to the timer 18, which constitutes means for measuring elapsed time. Thereupon the equipment remains in operation until 100 pulses are received by the register-switch 16a, at which point the timer power source 26 is inactivated. The timer 18 accordingly sets the preset time switch to time an interval equal to the time required for the detector itl to produce a number of pulses equal to 1GO times the scaling factor of 'the sealer l2. Thereupon the detector it? is exposed to the sample under measurement with the switch 2d in the position to feed power to the time switch 2G. The equipment operates in this condition for the time set on the time switch 2l), at the expiration of which the sealer l2, the register-switch lon, and the source of timer power 2e are all inactivated. At this point the indication of the register-switch which is the quotient of the number of pulses occurring divided by the scaling factor', constitutes a direct indication of the percentage ratio of the radioactivity of the unknown specimen to that of the standard specimen.

lt will of course be understood that the device or" Figure 3, may r ed from standard units y be const of manuta tu which are familiar to those skilled in the art, a variety of speciiic forms of the circuit elements indicated in bloei; form being iable and familiar. ln Figure y there is illustrated particular electrical systeni w ich may employed to provide the timer, the timer power, and the preset time switch of Figure 3, together with certain of the couplings shown in dotted form in Figure 3. the system of Figure 4, the output of a scintillation counter 345-, corresponding to the detector l@ of Figure 3, coupled to a pulse equalizer (and ampliiier) the output of t' ich is in turn coupled to a scale-of-l, 'i signated 33, w ich is a four-stage binary scaier, the latter being coupled in Lirn to a predetermined count scale-of-lGG designated As is conventional in predetermined count Sealers, a relay having a winding -t-Z and normally open contacts nl is actuated upon the .recording of l0() counts on the predetermined count sealer As indicated by the dotted lines d6, the closing of the contac A 5 puls tively coup a multi-position pul Y or to the ally designated e switches and :33 may be or' the type commonly luown as stepping switches]7 which are commonly available from many manufacturers of telephone relays, and which have large numbers of stationary contacts, the number' of contacts being further increased by suitable 'erconnection of successive switch banks in the man which is conventional in employnient of such s. hes. it will of course be understood that the relatively small number of stationary contacts oil and which are illustrated consti te only a small portion of the number or contacts actually incorporated in the type of switches so schematically indicated. In one embodiment of the invention, the switches and 53 are 40G-position switches, having corresponding centacts directly interconnected, as shown. The rotor contacts and 66 of the respective switches are connected in series with a power source et? and a relay winding 70, the contacts associated with the winding fil being connected in parallel with the contacts dd, described above. As shown by the dotted line 7d, the closing of either the contacts ifor the contacts 72 stops the operation of the pulse generato in addition to stopping the operation of the scalers and as described above. The initial positions of the rotor contacts 64 and 66 are such as to maite contact with the Xed contacts oil and 62 which are uppermost in the drawing, and which are otherwise unconnected. rhe detector is exposed to the standard specimen with the switch $9 in the position wherein the pulses from the pulse generator" are fed to the winding 52, thus advancing the rotor contact 64!- one position (in the direction indicated by an arrow) each one-fifth of a second, this advancement of the rotor 6e continuing until 1600 pulses have been fed into the sealer 33, at which time the equipment, including the pulse generator L53, is inactivated described above by the closing of contacts T he switch Sd is then thrown to the position wherein the rotor 56 is driven (in the direction indicated by an arrow) by the pulses from the pulse generator f-, the motor e4.- being disabled and remaining in the position previ ly reached, the counter now being exposed to the unknown specimen under measurement. At this point, therefore, the rotor contact 6e now moves forward one position each one-fth of a second, and this operation continues until the rotor 66 reaches the position corresponding to that occupied by the rotor 6d, at which time the relaf, winding 7d is energized and the counting operation and the operation of the switch 53 are accordingly terminated. The reading of the sealer lil is a direct indication of the percentage ratio of the activity or the specimen under measurement to that of the previously measured standard (the former of course being smaller than the latter). In the embodiment illustrated in Figure 4, there is provided a switch 76 mechanically coupled to the switch 50, the switch 76 automatically resetting the scalers 33 and dit each time the position of the switch 59 is chanoed.

In accordance with the patent laws, there are above described, and schematically illustrated in the drawing, a number of embodiments of the method and apparatus of the invention. Obviously, those skilled in the art will readily devise variants which utilize the basic teachings of the invention. Accordingly, the scope of the monopoly to be afforded the invention shall be determined, not from the particular embodiments described herein, but solely from the appended claims.

What is claimed is:

1. Apparatus for indicating the percentage ratio of the radioactivity of a radioactive specimen to that of a standard specimen comprising, in combination, a radiation detector adapted to produce electrical pulses proportional in rate to radioactivity, a preset time switch adapted to be set to a plurality of selectable timing intervals, means coupled to the detector to set the preset time switch for a timing interval equal to the time required for the occurrence of a number of pulses from the detector equal to a multiple of 100, and means responsive to the preset time switch and to the detector to register the quotient of the number of pulses from the detector occurring during such timing interval divided by the multiplier of said multiple.

2. Apparatus for indicating the percentage ratio of the radioactivity of one radioactive specimen to that of a standard specimen comprising, in combination, a radiation detector adapted to produce electrical pulses proportional in rate to radioactivity, elapsed time measuring means, means for operating the elapsed time measuring means only during the occurrence of a number of pulses responsive to the standard equal to a multiple of 100, a preset time switching means, means responsive to the elapsed time measuring means to set the switching means for a timing interval equal to that required for said occurrence, and means to register the quotient of the number of pulses occurring from the sample under measurement during the timing interval so set on the switching means divided by the multiplier of said multiple.

3. Apparatus for indicating the percentage ratio of the radioactivity of a radioactive specimen to that of a standard specimen comprising, in combination, a radiation detector adapted to produce electrical pulses proportional in rate to radioactivity, a non-decimal sealer coupled to the detector, a decimal scaler coupled to the rst scaler, a preset time switching means, means responsive to the decimal sealer to set the preset time of the switching means equal to the time required for one complete cycle of the decimal scaler when the detector is exposed to the standard specimen, and means for coupling the switching means to at least one of the scalers so that subsequent exposure of the detector to the unknown specimen produces on the decimal sealer a direct indication of the percentage ratio.

4. A method of indicating the percentage ratio of the radioactivity of one radioactive specimen to that of a standard specimen of at least equal activity comprising the steps of generating pulses proportional in rate of occurrence to the radioactivity ot the standard, registering the quotient of the number of pulses occurring divided by a constant factor until this quotient reaches 100, and thereupon generating pulses proportional in rate of oc` currence to the radioactivity of the unknown specimen, and registering the quotient of the number of these pulses occurring during the time previously required for the first registering divided by the same constant factor, whereby the second quotient is equal to the percentage ratio of the activity of the unknown specimen to that of the standard.

5. Apparatus for indicating the ratio of the radioactivity of an unknown specimen to that of a standard speciment comprising means including a radiation detector and Scaler for producing electrical pulses proportional in rate of occurrence to radioactivity, a rst inactivating means coupled to the pulse-producing means for inactivating the pulse-producing means upon receipt therefrom of 10" pulses, where n is an integer greater than unity, a second inactivating means coupled to the pulse-producing means for inactivating the pulse-producing means on the expiration of a presettable time interval, a coupling between the tirst and second inactivating means constructed and arranged for presetting the presettable time interval of the second inactivating means, and manually operable switch means to selectively activate and inactivate the presetting coupling for measurements on the standard and unknown specimens, respectively, whereby the number of pulses produced by the pulse-producing means during the time interval so preset is a direct indication of the percentage ratio of the radioactivity of an unknown specimen to that of a previously measured standard speci-` men.

6. Apparatus for indicating the percentage ratio o? the radioactivity of one radioactive specimen to that of a standard specimen comprising, in combination, a radiation detector adapted to produce electrical pulses proportional in rate to radioactivity, an elapsed time timer, means for operating the timer only during the occurrence of a number of pulses responsive to the standard equal to a multiple of 100, a preset time switching means, a coupling between the timer and the switching means constructed and arranged for setting the switching means for a timing interval equal to that required for said occurrence, and means to register the quotient of the number of pulses occurring from the sample under measurement during the timing interval so set on the switching means vdivided by said multiple.

7. Apparatus for indicating the percentage ratio of the radioactivity of one radioactive specimen to that of a standard radioactive specimen comprising, in combination, a radiation detector adapted to produce electrical pulses proportional in rate to radioactivity, means to measure the time required for the occurrence of a predetermined number of such pulses, such number being a multiple of l0, where n is an integer greater than unity, means to indicate numerically the number of pulses occurring in a presettable time divided by the multiplier of said multiple, and a coupling between said predetermined number means and said presettable time means constructed and arranged for the setting of said presettable time equal to said rst time.

8. Apparatus for indicating the percentage ratio of the radioactivity of a radioactive specimen to that of a standard specimen comprising, in combination, a radiation detector adapted to produce electrical pulses proportional in rate to radioactivity, counting apparatus comprising a sealer coupled to the detector and a register coupled to the sealer, a preset time switching means, circuit elements interconnecting the counting apparatus and the switching means and constructed and arranged to set the preset time of the switching means equal to the time required for the registering of 1G pulses on the register, where it is an integer greater than unity, when the detector is exposed to the standard specimen, and means for coupling the switching means to the counting apparatus, so that subsequent exposure of the detector to the unknown specimen produces on the register a direct indication of the percentage ratio.

9. Apparatus for indicating the percentage ratio of the radioactivity ot a radioactive specimen to that of a standard specimen comprising, in combination, a radiation detector adapted to produce electrical pulses proportional in rate to radioactivity, counting apparatus comprising a sealer coupled to the detector and a register coupled to the sealer, a preset time switching means including a movable presetting member connected to set the preset time of the switching means, a drive connected to the movable member, a connection between the drive and the register constructed and arranged to inactivate the drive upon the registering of 10" pulses on the register in response to a standard specimen, where u is an integer greater than unity, a switch independent of the register in 10 circuit with the drive to disable the drive for measurements on unknown specimens, and means for coupling the preset time switching means to the counting apparatus.

References Cited in the tile of this patent UNITED STATES PATENTS 2,585,649 Hanson Feb. l2, 1952 2,610,303 Bell Sept. 9, 1952 2,685,027 Alvarez July 27, 1954 2,695,364 Wolfe Nov. 23, 1954 

5. APPARATUS FOR INDICATING THE RATIO OF THE RADIOACTIVITY OF AN UNKNOWN SPECIMEN TO THAT OF A STANDARD SPECIMEN COMPRISING MEANS INCLUDING A RADIATION DETECTOR AND SCALER FOR PRODUCING ELECTRICAL PULSES PROPORTIONAL IN RATE OF OCCURRENCE TO RADIOACTIVITY, A FIRST INACTIVATING MEANS COUPLED TO THE PULSE-PRODUCING MEANS FOR INACTIVATING THE PULSE-PRODUCING MEANS UPON RECEIPT THEREFROM OF 10N PULSES, WHERE N IS AN INTEGER GREATER THAN UNITY, A SECOND INACTIVATING MEANS COUPLED TO THE PULSE-PRODUCING MEANS FOR INACTIVATING THE PULSE-PRODUCING MEANS ON THE EXPIRATION OF A PRESETTABLE TIME INTERVAL, A COUPLING BETWEEN THE FIRST AND SECOND INACTIVATING MEANS CONSTRUCTED AND ARRANGED FOR PRESETTING THE PRESETTABLE TIME INTERVAL OF THE SECOND INACTIVATING MEANS, AND MANUALLY OPERABLE 